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Tuesday, October 18, 2011

My JBOT 5 Watt Linear (Farhan's Design)

There it is -- my version of Farhan's JBOT 5 watt linear amplifier. JBOT stands for "Just a Bunch of Transistors." But I think Farhan is being modest. I really like the design, especially the effort to make this something that hams around the world can reproduce from readily available parts. For example, Farhan's original version used TV balun cores for the transformers. I didn't have any of those around, but I found three "mystery spec" binocular cores in my junk box. They seem to be working just fine. (I tested them a bit: Farhan had written that FT 37-43 cores would work. T1 has seven turns trifilar. Seven turns on my mystery cores yielded 13uH. On an FT 37-43 core 7 turns yields around 20 uH. Close enough -- these are, after all, broadband transformers.)

Note how closely my build follows Farhan's schematic (which you can see in the background). When building this circuit, I just kept Farhan's web page on my computer screen, and scrolled up and down from his schematic to the photo of his version.

This is the first linear amplifier that worked the first time I powered it up. It didn't release any smoke, or leave transistor burn tattoos on fingers, or try to be a 14MHz oscillator.

This version is going into my Azores-built 17 meter DSB transceiver. See how nice it fits:

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  1. Great work Bill!

    Was gonna say how alike your layout was to Farhan's and then you explained that you had been following Farhan's photo!

    I really want to have a go at this circuit, there is only one problem - I don't have a phone rig to drive it with (yet.......)


  2. Well done Bill. I can definitely see that the solder has been melting up in NoVa.

    Hope we can work one another someday. I'm still building my Farhan Bitx20 and hope to get it on the air real soon.

  3. Nice build. But this amp is showing its age. The 2N2218/2N2218A's or 2N2219/2N2219A's in TO-39 cans are getting pretty hard to find (but it is stil possible). Anyone have a suggestion for a newer and better replacement part - in an easier to find and easier to cool package? It would breath new life into this amp.

    David WB4ONA

  4. I found a nice video of this amp being tested by Tom AK2C here:


    Tom used 2N3053 transistors for the two input devices and 2SC2166's for the four in the output stage. Still, I'd like to find easier to find transistors though.

    73's David WB4ONA

  5. Fits nice in the Azores 17. How much power are you making? Uh, oh. Where's the output filter? ;-)

    73.......Steve Smith WB6TNL
    "Snort Rosin"

  6. Steve: Re the output filter. Obviously you aren't aware of the special FCC rule that permits operation without output filter IF there is no room for said filter in the cabinet or on the chassis. It's a rule! Look it up!
    BTW: We have one more thing in common: Washington Heights. I lived in Inwood until I was 8. My dad was a cop in the 34th Precinct.
    73 Bill "we-don't-need-no-stinkin'filters" Meara


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